It's possible that it is measuring internal traffic data, i.e; traffic moving from the host to the machine. Additionally, if you are running any rsync processes, acd_cli processes, etc. It will read those stats as well to be traffic on the adapter.
Check it with the vnstat command
vnstat -l to ensure that it matches that of the displayed graph on the dashboard. By default vnstat live via cli measures in Mbits, so just keep that in mind when running it for comparisons. More than likely though, there is some sort of internal traffic happening between host and machine; however, this usually happens on l0 and not eth0.
Install the utility called nethogs for further digging. This can be installed with
apt-get install nethogs. Then run it with
nethogs eth0. This will give you a very clear rundown of what processes are using how much bandwidth.